Our objective is to understand quantitatively the control of development in the cellular slime molds. We are attempting to do so by a joint theoretical and experimental effort using such experimental tools as precision time-lapse cinemicrophotography, artificial signal sources, analysis of mutants defective in developmental competences, and grafting of the tip, or organiser. The theoretical techniques, though specifically biological in nature, resemble in style those employed in physics. By thus analyzing a relatively simple system, we hope to gain insight into development at the cellular levels and its control at the multicellular level in more complex embryos. We are concentrating on the aggregation process in Dictyostelium discoideum, and the period of differentiation, interphase, during which the competences required for aggregation are developed. Measurements on the wild type are complemented by studies of mutants defective in one or more of these competences.